1, 25x12= 25x10+25x2
= 250+50
= 300
2, 37x102=37x100+37x2
= 3700 + 74
= 3774
3, 125x24=125x20+125x4
= 2500+500
= 3000
4, A= 1+2+3+...+50
= [(50-1)/1+1]x(50+1)/2
= 1275
Tiếp tục :
5,
B=1x2+2x3+3x4+...+20x21
3B=1x2x3+2x3x3+3x4x3+...+20x21x3
3B=1x2x3+2x3x(4-1)+3x4x(5-2)+...+20x21x(22-19)
3B=1x2x3+2x3x4-1x2x3+3x4x5-2x3x4+...+20x21x22-19x20x21
3B=20x21x22
3B=9240
B= 9240/3
B=3080
k nha